Seismicity of Iceland

1. Seismicity of Iceland Dr. Benedikt Halldorsson

2. Earthquake Occurrence in Iceland • Overview • Tectonics • Interplate Earthquakes • Transform Zones • South Iceland Seismic Zone • Tjörnes Fracture Zone (North Iceland Seismic Zone) • Volcanic Zones • Reykjanes Peninsula • Hengill Triple-junction • Eastern Volcanic Zone • Intraplate Earthquakes • Induced earthquakes • Earthquake hazard map

3. Iceland and Plate tectonics • Horizontal motion of plates Steve Gao

4. Iceland and Plume tectonics • Large scale vertical mantle plume motions • Hot-spots are one manifestation of plume tectonics USGS

5. Tectonics of Iceland: Combination of plate and plume tectonics The “Icelandic Mantle Plume” (DrDietmar Müller, University of Sydney) Lundin & Dore (2004)

6. Tectonics of Iceland: Combination of plate and plume tectonics • Mid-Atlantic Ridge of tectonic extension between the North American and Eurasian Plates. • RR=Reykjanes Ridge • KR=Kolbeinsey Ridge • In Iceland the interplay between the tectonic extension and mantle plume define the geodynamics • Including volcanic and earthquake occurrence (Maclennam, 2001; Kaban et al., 2002)

7. Topography of Iceland Björnsson & Palsson (2008)

8. Volcanic Systems and Earthquake Epicentres in Iceland • Red dots = earthquake epicentres 1994-2005 • Volcanic systems • Black circles = central volcanoes • Yellow regions = fissure swarms • White regions = glaciers Guðmundsson (2001)

9. Present-Day Geodynamics of Iceland • Average horizontal velocities from GPS measurements • Green=NS-component • Red = West component • Blue= East component • Defines the Present-Day Rift Axis of the Mid-Atlantic Ridge in Iceland Sigbjörnsson et al., 2006

10. Main tectonic structures and earthquake epicentres

11. Simplified Geological Map of Iceland Guðmundsson et al (2008)

12. Population Density in Iceland Guðmundsson et al (2008)

13. The Icelandic Strong-motion Network

14. The South Iceland Seismic Zone • Strong-motion stations in South Iceland • Monitor ground movements and building response in the region

15. Seismicity in the South Iceland Seismic Zone [Einarsson, et al.] Stefansson et al. (2006)

16. The Tjörnes Fracture Zone(North Iceland Seismic Zone) • Strong-motion stations in North Iceland • The Tjörnes Fracture Zone is largely offshore

17. Seismicity in the Tjörnes Fracture Zone • Recent strong historical earthquakes (Green stars) • Microearthquakeepicentres 1994-2012 (Black dots) • Volcanic systems • Central volcanoes (Black circles) • Fissure swarms (red shaded regions) • Rift axis (red dashed lines) and direction of rifting 7.0 6.2 7.0 6.5 6.5 6.3 7.0 6.5 6.2 IMO (2012)

18. Ongoing earthquake sequence since 2012 in the Tjörnes Fracture Zone • Activity on the two major structures of the TFZ, and the extension ridge • Largest events so far are M5.5 and M5.6 • Measured by the new strong-motion array in Husavik, North Iceland IMO (2013)

19. Seismicity in the Reykjanes Peninsula and the Hengill Triple Junction • Reykjanes Peninsula • Narrow seismic zone with shallow focus earthquakes • Normal faulting • Hengill • Triple junction betweenthe Reykjanes VolcanicZone, The WesternVolcanic Zone, andthe South Iceland Seismic Zone. • Seismicity of the Hengill volcanic system, normal faulting and seismicity associated with geothermal activity South Iceland Seismic Zone Reykjanes Peninsula

20. Volcanic Earthquakes in Iceland • Rifting structures mostly aseismic except during eruptions • Spatial clustering of epicenters – central volcanoes • Primary classes of seismicity of the volcanic zones • Rifting earthquakes • Inflation earthquakes • Deflation earthquakes • Intrusion tremors • Eruption tremors • Relatively smaller magnitudes than in the transform zones

21. Intraplate Earthquakes in Iceland • Very rare events • Primarily two cases • Earthquakes in west Iceland • Borgarfjordur events of 1974 • Largest magnitude was 6 • No apparent single fault • Normal faulting • Earthquakes on the insular shelf off Eastern Iceland • Most located near shelf-edge

22. Induced Seismicity due to FluidInjectionHellisheidi Geothermal Power Plant, Southwest Iceland Epicenters of induced earthquakes due to fluid injection at Hellisheiði Power Plant (45 days) Station locations of the ICEARRAY I in Hveragerði, 11 km away from the Power Plant • The induced seismicity culminated in two ML 3.8 earthquakes on 15 October 2011 Halldorsson et al. (2012)

23. Earthquake Hazard in Iceland 10% probability in 50 years of exceeding the specified Peak Ground Acceleration We are here! (Solnes, Sigbjörnsson & Elíasson, 2004)

24. Summary • Iceland is one of the most active countries in the world in terms of seismicity and volcanism • Its seismicity is caused by a complex interaction between the tectonics and volcanism of Iceland • The two having different geodynamics and manifestation • The largest earthquakes in Iceland occur in the two transform zones in the south and north, respectively • Style of faulting depends on the development of the fault lineament • The SISZ exhibits “bookshelf tectonics” • The SISZ constitutes a unique natural field laboratory for the study of earthquakes, their strong-motions and their effects on the built environment